This is not a research level question, but it follows immediately from the NP-hardness of 3SAT. The old book "Computers and Intractability" by Garey and Johnson used to be (i.e. there might be something better now) a good place to learn this topic. It's not something that can be explained in a few paragraphs. cs.stackexchange.com (the computer science stackexchange) is a better place for this type of question if you need further help.